1. Field of Invention
The present invention relates to compounds that are analogs of SR141716A having unique CB1 receptor interactions and pharmacological profiles and their use in treatment of a variety of disorders such as substance abuse, obesity, schizophrenia, and memory dysfunction.
2. Discussion of the Background
The use of Cannabis sativa, Cannabis indica, and cannabinoid preparations in medicinal, religious, industrial, and social settings has an extensive history, with the first recorded medicinal use occurring in 2737 B.C. Despite its recognition and use as a medicinal natural product by many cultures, including that of the aboriginal in the United States, its use in western medicine began to decrease early in this century. With the passing of the Marijuana Tax Act of 1937, its cultivation and use were effectively prohibited in the U.S. In 1942, the U.S. Pharmacopeia removed marijuana from its listing. Despite marijuana usage falling into disfavor during this time, the medicinal properties of cannabis continued to be investigated. These studies were directed toward both therapeutic applications and the understanding of the mechanism(s) of action. As a result, the primary psychoactive constituent xcex949-tetrahydrocannabinol (THC) was identified, and the structure-activity relationships (SARs) of the cannabinoids were explored (Mechoulam et al., 1970). These studies ultimately resulted in the classification of the cellular effects of a wide variety of cannabinoids in cellular and laboratory animal test systems (Martin, 1986). SARs were generated in man and laboratory animals, particularly with regard to psychotomimetic and analgesic activity (Razdan et al., 1986). Although some studies have shown therapeutic utility in the treatment of cancer chemotherapy nausea, glaucoma, and other disorders, the only therapeutic application for cannabis or cannabinoids with FDA approval is the use of Marinol(copyright) (xcex949-THC in sesame oil) as an anti-emetic.
The continued synthesis and identification of novel cannabinoids, particularly within the last 20 years, has provided researchers with a variety of chemical probes that have facilitated a rapid expansion in the knowledge of the neurochemical substrates and mechanisms of action of cannabis and cannabinoids. The discovery of the nonclassical cannabinoids (Johnson and Melvin, 1986) and the use of the bicyclic cannabinoid [3H]CP55,940 as a high affinity ligand enabled the identification, localization and molecular characterization of cannabinoid receptors (Devane et al., 1988; Herkenham et al., 1990; Matsuda et al., 1990) and did much to initiate and sustain this renewed interest in cannabinoid research. Indeed, the discovery of other classes of cannabimimetic compounds such as the aminoalkylindole (WIN55212-2) and, more recently, the endogenous cannabinoid anandamide (arachidonylethanolamide; Devane et al., 1992a), were in some ways dependent upon the discovery of [3H]CP55,940. These compounds and others have resulted in the discovery of additional cannabinoid receptors, with the predominant form in the central nervous system (CNS) designated the CB1 site and the form found primarily in the periphery denoted the CB2 site (Munro et al., 1993). Similarly, the identification of the CB2 receptor has fostered the synthesis and characterization of receptor-selective cannabinoid ligands, such as 1-deoxy-11-hydroxy-xcex948-THC-DMH (Huffmann et al., 1996). All of the varied structural classes of cannabinoid ligands have been examined for their selectivity at CB1 and CB2 receptor systems (Showalter et al., 1996; Felder et al., 1995) and their influence on the second messenger systems coupled to these receptor subtypes (Howlett et al., 1988; Bayewitch et al., 1995) and the endogenous neurochemicals and enzymes (Deutsch and Chin, 1993; Childers and Deadwyler, 1996) involved in cannabinoid activity. In addition to providing high affinity ligands and novel tools for examining cannabinoid mechanisms, these compounds have also provided new templates for drug discovery.
More recently, Rinaldi-Carmona et al. (1994) reported SR141716A (having the structure of Formula (I)), a potent cannabinoid antagonist with nanomolar affinity that represents a unique chemical tool for further characterizing the cannabinoid receptor system in the CNS. 
That is, prior to the discovery of SR141716A, the identification of an antagonist for the cannabinoid receptor remained one of the final undiscovered pharmacological tools for further elucidating the mechanism of action and pharmacological relevance of cannabis and cannabinoids. While some compounds, such as cannabidiol or xcex949,11-THC (Beardsley et al., 1987), had previously been reported to have antagonist activity, their potencies were extremely low. More recently, other compounds were purported to be cannabinoid antagonists, such as WIN56098, WIN54461 (Eissenstat et al., 1995) and AM630 (Pertwee et al., 1995a); however, these molecules are also less potent than SR141716A and, in the case of WIN56098, have not been convincingly shown to be antagonists (Pacheco et al., 1994). SR141716A was the first compound reported to be both an antagonist in vitro and sufficiently potent in vivo to produce a withdrawal syndrome in cannabinoid tolerant animals (Aceto et al., 1995; Tsou et al., 1995). SR141716A has also been demonstrated to have therapeutic potential in treating obesity, both in laboratory animal studies (DiMarzo et al., 2001), and in human obese males (Le Fur et al., 2001). Furthermore, SR141716A blocked acute psychological and physiological effects of smoked marijuana without altering THC pharmacokinetics (Heustis et al., 2001). Therefore, cannabinoid antagonists constitute an additional family of cannabinoid receptor ligands that are currently being considered as rational compounds for pharmacotherapeutics and structure-activity relationship analyses. It is of interest to us and other cannabinoid researchers to determine whether these compounds interact within the same recognition site on the cannabinoid receptor and whether the population of neuronal receptor sites to which SR141716A binds is the same as that with which classical and nonclassical cannabinoids interact.
Receptor-Binding Properties of SR141716A
It is generally accepted that there are two types of cannabinoid receptors: CB1 and CB2. CB1 receptors are primarily expressed in the CNS and to a lesser extent in selected tissues of the periphery; CB2 receptors have been suggested to be limited to peripheral tissues. [3H]SR141716A in vitro has high affinity for CB1 (Rinaldi-Carmoni et al., 1994; Rinaldi-Carmoni et al., 1995a; Petitet et al., 1996; Hirst et al., 1996; Thomas et al., 1997) and binds to CNS receptor populations with the same pattern of distribution (Rinaldi-Carmona et al., 1995b) as that observed with CP55,940 (Herkenham et al., 1990), 11-OH-xcex949-THC-DMH (Thomas et al., 1992) and WIN55212-2 (Jansen et al., 1992; Kuster et al., 1993). This similarity of distribution provides further evidence that SR141716A binds specifically to neuronal cannabinoid receptors. SR141716A has marked selectivity for the CB1 over the CB2 receptor: the Ki for CB1 is over 50-fold lower (Rinaldi-Carmona et al., 1994; Felder et al.; 1995; Showalter et al., 1996). This degree of selectivity is relatively unparalleled among cannabinoid compounds. WIN55212-2, by comparison, has approximately 20-fold (Felder et al., 1995) or 7-fold (Showalter et al., 1996) greater selectivity for CB2, while CP55,940 has approximately equal affinity at these two receptor sites.
It is important to emphasize that the nature and locus of interaction of any cannabinoid ligand with the cannabinoid receptor is unknown. Receptor mutation studies using chimeras created between CB1 and CB2 receptor sequences have shown that alterations in the extracellular loop region between helices three and four of the seven transmembrane regions differentially affect the binding of SR141716A and CP-55,940, leading Shire et al. (1996) to conclude that the binding of these two compounds most likely involves different amino acids, if not different regions of the receptor. However, as the authors point out, modest changes in the three-dimensional conformation of receptors brought about by amino-acid substitutions in regions other than those involved in ligand recognition can alter the selectivity of a receptor (Fong et al., 1994). Therefore, there could be an overlapping receptor region that is capable of interacting with the antagonist SR141716A and cannabinoid agonists. Indeed, many antagonists share some but not all of the binding domain of the agonists, an idea supported by the observation that antagonists are frequently larger than agonists (Kenakin, 1993).
In-Vitro Effects of SR141716A on Signal Transduction Systems
SR141716A blocks cannabinoid-induced (CB1-coupled) inhibition of adenylate cyclase (Rinaldi-Carmona et al., 1994; Hirst et al., 1996), the predominate signal transduction mechanism of cannabinoids in the CNS. Because of its low affinity for the CB2 receptor, it is a poor antagonist of the CB2 receptor mediated inhibition of cAMP accumulation (Felder et al., 1995). The effects of SR141716A on cannabinoid-mediated modulation of potassium and calcium channels have not received as much study as the adenylate cyclase system. However, anandamide directly inhibits Shaker-related potassium channels that are found ubiquitously in the mammalian brain, as does xcex949-THC, and the inhibition occurs through a pertussis toxin-insensitive mechanism and is not prevented by SR141716A (Poling et al., 1996). In tissue preparations, SR141716A reverses cannabinoid-mediated inhibition of long-term potentiation in rat hippocampal slices (Terranova et al., 1995), contractions of mouse vas deferens (Rinaldi-Carmona, et al., 1994), electrically evoked contractions of the guinea-pig myenteric plexus-longitudinal muscle preparation (Pertwee et al., 1996a) and electrically invoked contractions of mouse urinary bladder (Pertwee et al., 1996b).
In-Vivo Effects of SR141716A
SR141716A blocks several of the primary pharmacological effects of cannabinoid agonists in laboratory animals. Pretreatment of mice with SR141716A prevents cannabinoid agonists from producing hypothermia, catalepsy, analgesia and decreased locomotor activityxe2x80x94the xe2x80x9cmouse tetradxe2x80x9d of cannabinoid effects (Compton et al., 1996). SR141716A antagonizes xcex949-THC""s discriminative stimulus properties in pigeons (Mansbach et al., 1996), rats, and rhesus monkeys (Wiley et al., 1995). Perhaps even more indicative of the potency of this antagonist is its ability in the 1-10 mg/kg range, to precipitate a withdrawal syndrome characterized by disorganized patterns of sequences of motor behavior in rats chronically treated with THC (Aceto et al., 1995; Tsou et al., 1995). These results have led some go investigators to suggest that because the psychotomimetic effects of cannabinoids can be blocked in laboratory animals, SR141716A should be capable of blocking, and perhaps reversing, cannabis intoxication in man. In addition to SR141716A, various analogs of SR141716A have been synthesized and shown to antagonize the in vitro effects of cannabinoid agonists and to bind to the same regions in the CNS (Gatley et al., 1996; Thomas et al., 1998; Rinaldi-Carmona et al., 1998), or in some instances act as partial agonists in certain cannabinoid assays (Houston et al., 1997).
Therapeutic Potential of Cannabinoid Antagonists
The potential therapeutic activities of SR141716A are in some ways dependent on the distribution of CB1 cannabinoid receptors in the CNS, the regions to which the receptor-invested neurons project, or to the cell-line that is being affected. For example, CB, receptors are localized in regions of the hippocampus, which would indicate that modulation of the cannabinoid system might alter the processing and storage of information (Herkenham et al., 1990). The globus pallidus and the substantia nigra pars reticulata are also heavily invested with cannabinoid receptors localized on the axon terminals of striatal efferent neurons, which suggests that alteration of this system might affect movement control. Cannabinoids are known to produce a number of cellular effects (Martin, 1986), and cannabinoid agonists exhibit a wide range of pharmacological activities in laboratory animals and man (Dewey, 1986). Because of the diversity of these effects, the therapeutic utility of cannabinoid antagonists could be quite wide-ranging. Indeed, it has recently been shown that anandamide levels in mouse uterus are associated with uterine receptivity for embryo implantation (Schmid et al., 1997). Furthermore, anandamide levels in the preimplantation mouse uterus are at the highest level yet determined in any mammalian tissue. Since anandamide and CP-55,940 both inhibit implantation, and this effect is reversed by SR141716A, cannabinoid antagonists could play a role in the treatment of early pregnancy failures or female infertility. Even dopamine release in guinea pig retina is inhibited by activation of cannabinoid receptors that appear to be tonically regulated by an endogenous ligand or are pre-coupled to the G-protein effector system (Schlicker et al., 1996). Thus, the relatively recent discovery of cannabinoid antagonists has led to the rapid identification and expansion of systems under cannabinergic control which represent potential therapeutic indications for cannabinoid modulation.
The antagonist activity of SR141716A in the CNS has led to speculation that this compound may be able to prevent or reverse cannabis intoxication in man. This activity could be useful in drug abuse intervention. For example, a depot form might prevent drug-seeking and relapse. The utility of SR141716A in understanding and treating drug abuse might extend beyond direct antagonism. An antagonist used as a probe could elucidate the biochemical basis of cannabis abuse, and substance abuse in general, and thus be useful in developing other substance abuse treatment modalities. Evidence in support of this contention can be summarized as follows: Cannabinoid receptors do not reside on mesencephalic dopaminergic neurons projecting to either the caudate-putamen or the nucleus accumbens (Herkenham, 1992). However, cannabinoid receptors are located in these regions, and cannabinoids elevate extracellular dopamine levels there (Ng Cheong Ton et al., 1988; Chen et al., 1990) and in other regions possessing cannabinoid receptors (Chen et al., 1993). Because drugs that elevate dopamine levels in the striatum, such as cocaine, have abuse liability in humans (Kornetsky, 1985), SR141716A might be expected to attenuate the abuse liability of cannabinoids and other abused substances through its ability to diminish their stimulation of dopaminergic activity in the brain""s reward circuitry. However, in behavioral studies of reward and aversion (Sanudo-Pena et al., 1997), cannabinoid agonists induced place aversion while cannabinoid antagonists induced place preference, a finding opposite to what one might anticipate based on the effects of these compounds on the striatal dopamine system. In rats, the concurrent administration of SR141716A during a 30-day chronic ethanol exposure increases the preference for ethanol; whereas the administration of the CB1 antagonist after chronic alcohol or at the time of withdrawal drastically diminishes the ethanol preference (Lallemand et al., 2001). Finally, in CB1 knock-out mice that do not respond to cannabimimetic agents, the acute effects of opiates are unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome are strongly reduced (Ledent et al., 1999). The diminution of opioid withdrawal in CB1 cannabinoid receptor knockout mice supports the notion that the cannabinoid system modulates dependence and withdrawal. Also consistent with the notion that cannabinoid agents can modulate withdrawal is the observation that SR 141716A administered repeatedly to morphine-dependent rats lessened the intensity of naloxone-precipitated withdrawal (Rubino et al., 2000).
SR141716A analogs might also have medicinal properties through their ability to modulate the cannabinoid system in the absence of a pre-existing effect produced by an exogenous compound such as xcex949-THC. In this instance, additional utility or medicinal potential for cannabinoid antagonists depends upon their ability to antagonize the effects of endogenous ligands, such as anandamide, which are contributing to a cannabinergic tone. Alternatively, constituitive coupling of cannabinoid receptors to G-proteins provides therapeutic potential for inverse agonists. Perhaps the most convincing demonstration of the therapeutic utility of cannabinoid antagonists or inverse agonists in the absence of exogenous agents was demonstrated in the report by Le Fur et al. (2001). In these studies, treatment of obese males with SR141716A resulted in a significant decrease in body weight that continued over the entire period of treatment. This was an obvious, albeit until then unproved, reversal of the ability of cannabinoid agonists, including endogenous agonists, to increase appetite. However, the therapeutic utility of cannabinoid antagonists has yet to be fully explored, and experimental studies of SR141716A and its effects on cannabinoid systems exhibiting tone continue to identify promising pharmacological activities (Collins et al., 1995; Lichtman et al., 1995; Lichtman and Martin, 1996, Terranova et al., 1996; Richardson et al., 1997; Smith and Martin, 1992; Smith et al., 1994; Compton et al., 1996; Pertwee and Fernando, 1996b). Thus, the discovery of cannabinoid antagonists has led to the rapid identification and expansion of systems under cannabinergic control which represent potential therapeutic indications for cannabinoid modulation.
Accordingly, there is a need to identify cannabinoid antagonists with increased CB1 selectivity, relative to SR141716A, to better target treatment regimens. It is also possible that compounds can be identified that selectively displace the various structural classes of cannabinoid ligands at the CB1 receptor. For example, a compound might fully displace [3H]CP55940 and [3H]SR141716A with reasonable affinity, while simultaneously much less able, or unable, to displace [3H]WIN55212-2 from the CB1 receptor in humans (hereafter referred to as xe2x80x9cWIN-sparingxe2x80x9d). These compounds might be anticipated to possess unique pharmacological properties, particularly since it is unclear whether the eicosanoids, anandamide, 2-arachidonylglycerol and 2-arachidonylglyceryl ether are the only, or even the primary, endogenous ligands, and it remains to be determined how these eicosanoid compounds interact with the CB1 receptor.